The notion of enjoying a cold one aboard a rush-hour commuter train leaving Manhattan may seem like a time-honored tradition, evoking visions of communal bar cars or perhaps a cocktail hour spent in solitude at one’s seat. But after midnight on weekends, the specter of alcohol aboard Long Island Rail Road trains carries a far more malevolent overtone; passengers described them as the “drunk trains,” characterized by fights and boisterous behavior. In March, passengers were accused of attacking two conductors.